Sickle cell anaemia is a recessive disorder: only people who carry two copies of the mutant allele (i.e. homozygous) have the symptoms.

People who are heterozygous (carry one mutant allele) for the sickle cell anaemia gene have some protection against malaria, yet do not have the full symptoms of sickle cell anaemia.

There is very high prevalence of sickle cell anaemia in populations from regions where malaria was historically common because the mutant allele confers an evolutionary advantage in these regions. Malaria has now spread to other areas such as South America, but the allele is not present in these populations.

Only 0.005% of babies are born with sickle cell anaemia in the UK. However, in countries such as Nigeria where malaria is endemic, this number can be as high as 2%.

The historic distribution of malaria is shown on the left, and the distribution of the sickle cell trait is shown on the right.

Cystic fibrosis is a life limiting disease caused by a mutant trans-membrane protein.

The protein usually transports chloride ions into the mucus that lines the cells.

These chloride ions are linked to water molecules. When the chloride ions are transported through the membrane, some of the water molecules become bound to the mucus, making the mucus more fluid.

The mutant transporter does not function correctly, so mucus becomes sticky and builds up. This prevents cells from functioning properly.

A build-up of bacteria in the mucus leaves the patient susceptible to infections.

The mutant protein is most commonly caused by a three base deletion that prevents the synthesis of the amino acid phenylalanine (Phe). There are over 1000 other rare mutations that in total make up 30% of cystic fibrosis cases.

Cystic fibrosis affects the lungs, liver, pancreas and the intestines. The damage to these organs is often so severe that many sufferers need organ transplants.